Many biological decisions are controlled by complex transcriptional circuits. Reagents that can specifically modulate such circuits would be extremely useful, as tools for dissecting the cis-regulatory logic of complex processes, and ultimately as therapeutics. The best developed tools are synthetic DNA binding proteins, but these lack the combinatorial specificity observed in natural transcription factors. Ultimately, more precise regulation could be achieved with sets of designer factors that act in concert. Many natural combinatorial transcription factors act as allosteric switches -- the factor is only active in the presence of another co-factor. Recently a novel strategy for introducing allosteric control over a protein activity has been developed. This strategy involves the introduction of conformational autoinhibitory interactions into a protein; lautoinhibition can be relieved by association with an appropriate partner protein. My specific aims are to use Ithis autoinhibitory strategy to generate synthetic transcription factors in which: 1) DNA binding activity is allosterically regulated by an adjacent DNA binding co-factor. 2) Transcriptional activation function is allosterically regulated by an adjacent DNA binding co-factor. These designer factors should yield extremely tight transcriptional control -- activation is dependent on the simultaneous presence of a precise combination of molecules.